// 生日悖论
pub fn birthday_paradox_probability(n: u32) -> f64 {
    if n >= 365 {
        0.
    } else {
        let n = n - 1;
        1. - (365 - n..=364).map(|x| x as f64).product::<f64>() / (365f64).powi(n as _)
    }
}
